The control of undesirable emissions such as oxides of nitrogen (NOx), hydrocarbons (HC) including volatile organic compounds (VOC), and carbon monoxide (CO) that are generated by power producers such as automobiles and electrical power generating stations is a well-studied field. The Background section of U.S. Pat. No. 5,891,409 provides a useful summary of the conditions and chemistries that produce such emissions and the approaches used to limit the release of these pollutants to the environment.
One technology for the control of oxides of nitrogen that is currently being used commercially at large land-based electrical power generating stations is selective catalytic reduction (SCR). The flue gases from a power station have a net oxidizing effect due to the high proportion of oxygen that is provided to ensure adequate combustion of the hydrocarbon fuel. Thus, the oxides of nitrogen that are present in the flue gas can be reduced to nitrogen and water only with great difficulty. This problem is solved by selective catalytic reduction wherein the flue gas is mixed with anhydrous ammonia and is passed over a suitable reduction catalyst at temperatures between about 150-550° C., and preferably between 300-550° C., prior to being released into the atmosphere. The ammonia is not a natural part of the combustion exhaust stream, but rather, it is injected into the exhaust stream upstream of the catalyst element for the specific purpose of supporting one or more of the following reduction reactions:4NH3+4NO+O2→4N2+6H2O  (1)4NH3+2NO+2NO2→4N2+6H2O  (2)8NH3+6NO2→7N2+12H2O  (3)4NH3+4NO+O2→4N2+6H2O  (4)Reducing agents other than ammonia, such as for example hydrazine, methyl hydrazine, monomethyl amine, and urea, or mixtures thereof, or mixtures thereof with ammonia, may also be employed in the processes described herein.
It is also known to combine an SCR process with a catalytic oxidizing process to treat an exhaust gas flow by oxidizing carbon monoxide to carbon dioxide and by oxidizing hydrocarbons to carbon dioxide and water. The oxidizing process is typically located upstream of the ammonia injection location and upstream of the reducing catalyst because the oxidizing catalyst will also function to oxidize ammonia, which is undesirable when it decreases the amount of ammonia available for reduction of the NOx and because it produces additional NOx compounds. U.S. Pat. No. 5,589,142 describes an emissions abatement system where an emission steam is passed sequentially through a first oxidizing catalyst, an ammonia injection location, a reducing catalyst, and then a second oxidizing catalyst. In this process, the amount of ammonia that is injected is controlled as a function of the NOx concentration and is specifically limited to a stoichiometric value. Thus, no excess ammonia is present in the emission stream as it leaves the reducing catalyst and there is no concern about generating additional NOx compounds in the trailing second oxidizing catalyst.
Modern air quality regulations mandate continuingly reduced emission levels for power generating plants, while at the same time fuel efficiency requirements continue to increase. Combustion controls alone may prove inadequate to satisfy these often-conflicting goals, and thus continued the improvement of post-combustion exhaust gas treatment systems is desired.